Method of operating a message receiver

ABSTRACT

A method of operating a message receiver for a message, that is present as a burst, which includes at least a training sequence and useful data is performed by stepwise synchronizing the receiver with the burst as the burst is received.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a national phase application of InternationalApplication No.: PCT/IB01/01760, filed Aug. 13, 2001.

The invention relates to a method for the operation of a messagereceiver for a message, that is present as a burst, that includes atleast a training sequence and useful data.

State of the Art

Such methods are known in the most diverse forms of embodiment. In anetwork with point-to-multi-point-access, which can be in the form of acellular network, bursts are used for message or data communication.These bursts are transmitted in fixed, sequential time slots accordingto the TDMA (time division multiple access) method known in the art.With such TDMA signals a burst includes at least a training sequence anduseful or signalling data, whereby the training sequence can come beforethe useful data. This training sequence is therefore also calledpreamble. However other TDMA methods are also known, in the case ofwhich the training sequence is transmitted in the middle of two usefuldata blocks as midamble. The training sequence can of course also beappended at the end of the useful data block.

The training sequence serves on the message receiver to enable thelatter to be synchronized with the burst so that data transmission ispossible from a base station to a terminal station and vice versa overan air interface. To synchronize the message receiver this must besynchronized at least with the carrier frequency and phase position ofthe carrier frequency of the burst. In addition it must be ensured thatthe receiver is synchronized with the fixed pre-set time slot, in whichthe burst has been transmitted, so that it is possible to demodulate anddecode the data sent.

In principle two types of synchronization are known, whereby in the caseof one type of synchronization the useful data stream is, in particularstatistically, evaluated. This type is also known as data supportedsynchronization. With another type synchronization takes place on thebasis of pilot signals, which are transmitted as a training sequence.

In the case of data supported synchronization the parameters forsynchronization are determined on the basis of statistical evaluationsof the transmitted or received signal. For this purpose various methodsare known, for example the “maximum likelihood sequence estimation”(MLSE). With another algorithm a closed control loop is used, in orderto be able to exactly calculate the synchronization parameters. Thisstatistical evaluation however consumes a great amount of time, so thatwith a high useful data rate to be transmitted this known method canonly be used with very great cost or complexity.

In particular for multi media systems, in the case of which images orvideo and audio sequences are transmitted in this way, the possibilityof synchronizing the message receiver is not so easy with the MLSEmethod, since it is virtually impossible to demodulate or decode theuseful data in real time.

Another type of data supported synchronization is based on a method ofViterbi & Viterbi, in the case of which the receiver is synchronized onthe basis of MPSK-modulated symbols by means of non-lineartransformation of the incoming message consisting of complex symbolswith mean value calculation and phase position determination. Thealgorithm used according to this Viterbi & Viterbi method requires apre-set interval of data, so that the mean value can be calculated. Adisadvantage with this method is that the phase position can only bedetermined if there is a slight frequency offset between transmitter andreceiver. In particular in the case of data transmission over airinterfaces however this is very seldom guaranteed, since multipathpropagation of the transmitted signals occurs and furthermore there isno frequency coupling between message receiver and transmitter.

With pilot signal supported synchronization very long training sequencesare communicated with the useful data, so that the synchronizationalgorithms can accurately determine the synchronization parameters.However as a result bandwidth for useful data communication is lost,since bandwidth is lost accordingly through the very long trainingsequences.

In GSM mobile radio communications useful data and signalling andcontrol information are also transmitted in accordance with the knownTDMA method. Due to the fact that essentially only speech istransmitted, a low data rate is the case for the useful data. Thephysical channel (time slot on a pre-set frequency) is divided intoseveral logic channels for transmission of the control and signallinginformation. That is to say in the case of a divided time slotsignalling and control information is transmitted once and with the nexttime slot useful data is transmitted in the same slot position. Atraining sequence is transmitted along with both the useful data and thesignalling and control information, which in GSM mobile radiocommunications lies in the middle between two useful data packets, andis therefore known as midamble. Because the physical channel, that is tosay the time slot on a frequency, is divided into a plurality of logicchannels, in which all the necessary data is transmitted, thetransmission rate for the useful data is however too low for multimediaapplications.

In the case of pilot signal supported synchronization methods it is alsoknown to use special training sequences in order to determine thecarrier frequency and sampling rate or clock frequency in the case ofburst transmissions. This method is based on a combination of filtrationand linear feedback of the received symbols within a time slot. Thistechnology is very similar to the previously mentioned pilot signaltechnology, in which all calculations are carried out in the timedomain. In the case of the known method now being described periodictraining sequences are transmitted, in which therefore unequal symbolsalternate at pre-set intervals. The received signal of this frequencycorresponds to a signal which is cosine in form. The Fourier transformof the received frequency provides two pilot signals. After the receivedsignal has been filtered and linear feedback carried out the symbol time(clock) and frequency offset can be deduced. This method has thedisadvantage that the transmission channel impulse response and timeslot synchronization cannot be carried out.

A type of synchronization is also known, in which for example two pseudonoise sequences are transmitted as midamble within a time slot.According to these evaluation algorithms synchronization is possibleboth for the carrier frequency and for the phase position, however fortime synchronization considerable complexity is needed.

Advantages of the Invention

With the method, which has the features stated in claim 1, it ispossible in an advantageous way as a burst, which includes includinguseful data and at least a training sequence, is received to synchronizethe receiver very rapidly with the burst. The receiver ispre-synchronized according to the invention, so that furthersynchronization parameters can be determined more rapidly, since thesymbols can be more easily evaluated by the pre-synchronized receiver.The total synchronization time is therefore reduced in an advantageousway. Therefore it is possible to use the message receiver in amultimedia transmission system in which a high data rate for the usefuldata has to be transmitted, so that images or video and/or audiosequences can be shown efficiently. The method according to theinvention can of course also be used in other transmission systems withhigh data transmission rates.

In a preferential form of embodiment, during its stepwisesynchronization in a first synchronizing stage, the receiver issynchronized with the clock of the message and the phase position of thecarrier frequency of the burst. This embodiment is preferred, if thefrequency offset is slight or negligible.

In another form of embodiment the receiver is synchronized during thefirst synchronizing stage with the clock of the message and the carrierfrequency of the burst. This variant is preferred if the phase offsetturns out to be slight.

In one form of embodiment the receiver is preferably synchronized duringa second synchronizing stage with the time slot of the burst. Thereceiver is therefore in this case synchronized with the physicalchannel.

In a refinement of the invention it is proposed that the transmissionchannel impulse response and/or automatic gain control (AGC) iscalculated during the second synchronizing stage. On the basis of thesesynchronization parameters the air interface, that is to say the radiotransmission path, can be monitored so that the channels can be aligned.

According to a further refinement of the invention it is proposed thatthe receiver is synchronized during the second synchronizing stage withthe carrier frequency of the burst. In particular this variant is usedif the clock and phase position have been determined during the firstsynchronizing stage. The receiver can thus be fine-tuned to the carrierfrequency during the second synchronizing stage.

If the clock and carrier frequency have been determined in the firstsynchronizing stage and if the receiver has been pre-synchronizedaccordingly, the receiver is synchronized with the phase position of thecarrier frequency during the second synchronizing stage. In particularpost synchronization of the phase position or carrier frequency isproposed, if the useful data has to be evaluated.

According to a further refinement of the invention it is proposed thatthe receiver is synchronized with the phase and/or frequency of thecarrier frequency after the first synchronizing stage and/or during orafter the second synchronizing stage. This synchronization sequence—alsoknown as tracking—is proposed in particular if the phase position and/orfrequency of the carrier signal vary during a burst. Tracking is thuscarried out in a preferred embodiment while the useful data is beingdecoded.

In a preferred form of embodiment a Viterbi decoder is used for trackingand decoding the useful data. Alternatively or additionally tracking canalso take place according to the Viterbi & Viterbi algorithms known inthe art. The Viterbi decoder or the Viterbi & Viterbi algorithms arecharacterized in particular in that preliminary decisions can be takenwith them, which finally make tracking possible.

In a particularly preferred embodiment it is proposed that the trainingsequence comprises at least two partial sequences. The partial sequencesare configured in such a way that allows gradual synchronization of thereceiver. Thus for example it can be proposed that one of the partialsequences is in itself periodic, as a result of which the phase positionand/or carrier frequency and/or symbol clock can be determinedparticularly simply and rapidly. In a preferred embodiment one of thepartial sequences is therefore evaluated during the first synchronizingstage, which is periodic in itself.

Furthermore in a preferred embodiment it is proposed that at least oneof the partial sequences comprises a symbol sequence, which allows thetime slot of the burst to be aligned and the channel impulse response,the phase position, the automatic gain control (AGC) as well astransmitting power to be calculated. For this purpose either partialsequences which include pilot signals can be proposed, or partialsequences which comprise pseudo noise sequences (PN sequences).Obviously combinations are also possible, so that one partial sequencecomprises pilot signals and the other partial sequence comprises thepseudo noise sequences. Naturally both partial sequences can comprisepilot signals or pseudo noise sequences.

In yet another embodiment it is proposed that for the secondsynchronizing stage the useful data is statistically evaluated. It wouldthen be sufficient for example only to transmit a partial sequence withpilot signals so that the first synchronizing stage can be carried out.The remaining synchronization parameters can then be determined bystatistical evaluation from the useful data received and the secondsynchronizing stage can be carried out.

A form of embodiment is preferred in which, irrespective of the numberof partial sequences, the training sequence comprises a constantduration or number of symbols. Thus the receiver can be easilysynchronized on the basis of the training sequence codes or symbolsequences known to it.

In a particularly preferred embodiment an incoming burst is detected bymeans of a power detector at the receiver input. Power detection thusserves as a quasi trigger (start) signal for starting the gradualsynchronization of the receiver.

According to a further refinement of the invention it is proposed thatthe time slot structure is determined with the incoming burst, that isto say the number of symbols in the useful data is evaluated. Thus itcan be easily determined whether useful data is actually transmitted, orwhether control or signalling information is transmitted, which usuallycomprises a lower number of symbols within the packet.

A preferred embodiment is one in which the channel impulse response iscalculated by cross correlation of the burst and the second partialsequence. In particular it is therefore proposed in this case thatcorrelation takes place between the useful data and the second partialsequence, from which the channel impulse response can be determined.

In a further refinement of the invention it is proposed that duringcross correlation the training sequence is shifted by a pre-definablenumber of symbols, in order to determine the corresponding number ofmultipath propagation echoes, from which the phase position andamplitude of each multipath propagation can then be determined in apreferred embodiment.

The AGC parameters can be determined in a particularly simple way fromthe phase position and amplitude of the particular multipath propagationechoes.

In addition it can be proposed that the constant phase position which isalso called the static phase position is determined for each multipathpropagation.

In another embodiment of the invention it is proposed that the periodicpartial sequence is evaluated in the time domain. Thus the pilot signalscan be determined, from which the frequency and/or phase position can beobtained.

The other partial sequence, which is used in particular for the secondsynchronizing stage, is evaluated in a preferred embodiment within thecomplex number range, so that the phase position and frequency can bededuced directly from the complex number pair having real and imaginaryparts.

In a particularly preferred embodiment the burst comprises the trainingsequence as preamble. Obviously it is however also possible to transmitthe training sequence as midamble. If at least two partial sequences aretransmitted with the burst as a training sequence, the two partialsequences can also be attached and/or inserted at different placeswithin the useful data.

DRAWING

The invention is described in more detail below on the basis ofexemplary embodiments with reference to the drawing, wherein:

FIG. 1 shows a data communication network with a base station andseveral terminal stations,

FIG. 2 a shows the structure of a burst,

FIG. 2 b shows different packet structures of a message for thetransmission from the base station to the terminal stations,

FIG. 2 c shows different packet structures for the transmissiondirection from a terminal station to the base station,

FIG. 3 as a block diagram shows a transmission chain from a terminalstation to the base station,

FIG. 4 shows a block diagram with the synchronization of a burst,

FIG. 5 shows a flow chart of a burst synchronization,

FIG. 6 shows a block diagram with the first synchronization stageaccording to a first embodiment,

FIG. 7 shows a block diagram of the first synchronization stageaccording to a second embodiment,

FIG. 8 shows a block diagram with the second synchronization stage,

FIG. 9 shows a block diagram with tracking according to a firstembodiment,

FIG. 10 shows a block diagram with the second synchronization stageaccording to a second embodiment, and

FIG. 11 shows the burst synchronization of a receiver in a summaryoverview.

DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

FIG. 1 shows by way of an extract a data communication network 1, whichcomprises at least one base station 2 as a network element and one ormore terminal stations 3, which can exchange data with the base station2 over an air interface 4. Data communication is also possible over theinterface 4 from the base station 2 to the terminal stations 3. Thus thebase station has at least one transmitter 5 and one receiver 6.Accordingly the terminal stations 3 are equipped with at least onetransmitter 7 and one receiver 8 in each case.

The base station 2 can be coupled to a network 9, which for example is apublic communications network. The transmission circuits in this network9 can be hard-wired or realized as air interfaces. The terminal stations3 can be connected to a network 10, whereby each of the networks 10 ispreferably a local network, to which a plurality of end-user devices(not shown) can be linked. The terminal stations 3 are arranged in fixedlocations in a preferred embodiment. It would however also beconceivable if the terminal stations 3 were configured as mobilestations. The base station 2 is preferably likewise in a fixed location.The data communication network 1 in a preferred embodiment is configuredas a cellular network, in which the terminal stations 3 can be arrangedat various distances from the base station 2. The data communicationnetwork 1 is in particular preferably used for transmitting multimediaservices, in which a correspondingly high data transmission rate has tobe achieved, in order to be able to provide multimedia services in realtime. For the transmission of data over this data communication network1 a burst mode is proposed, in which signal bundles known as “burst” aretransmitted over the air interfaces 4. These bursts are preferablytransmitted using TDMA technology, in which a plurality of time slotsare provided on a transmission or carrier frequency, whereby preferablyit is proposed that one of the terminal stations 3 has access to atleast one time slot.

In this so-called point-to-multi-point data communication network 1 withdivision of individual network sectors into cells, the base station 2for transmission preferably possesses one or more directional antennas,which are or can be directed toward the terminal stations 3. Severalfrequency channels for both transmission directions are feasible. Theindividual frequency channels preferably lie in a frequency band of 3.5to 28 GHz. Obviously other frequency ranges can be selected.

For data communication from the terminal stations 3 to the base station2 data is transmitted as bursts according to the TDMA method. Each ofthe terminal stations 3 for this purpose has at least one time slotassigned to it, in which a TDMA burst is transmitted, which isidentified in FIG. 2 a with the reference symbol 11. A TDMA burst 11 ofthis kind includes at least a useful data packet 12, at least a trainingsequence 13 and a protection time 14, during which nothing istransmitted, in order to prevent adjacent time slots being affected. Theburst 11 in this case comprises the following preferred structure: thetraining sequence 13 is transmitted as preamble 13′, that is to say itis sent before the useful data packet 12, whereby the useful data packet12 comes before the protection time 14. It would however also beconceivable according to another embodiment if the training sequence 13came between two partial useful data packets, i.e. transmitted asmidamble. Moreover it would be conceivable if the training sequence 13was appended to the useful data packet 12, and transmitted between theuseful data packet 12 and the protection time 14.

In order to allow a high data transmission rate from the terminalstations 3 to the base station 2, it is of utmost importance that thereceiver 6 in the base station 2 can be synchronized very rapidly withthe burst transmitted by the terminal stations 3, so that rapiddemodulation and decoding of the burst 11 are ensured, since the datatransfer rate depends on it in the end. In order to achieve this, in theembodiment according to the invention the receiver 6 of the base station2 is stepwise synchronized with the received burst 11. For this purposeit is proposed that the training sequence 13 is divided into two partialsequences 15 and 16, whereby the symbols contained in the two partialsequences 15 and 16 or the information reflected by the symbols is codedor modulated, in such a way that each partial sequence 15 and 16 can bedecoded and evaluated individually, so that the information contained inthe partial sequences 15 and 16 can be used for stepwise synchronizationof the receiver 6. The partial sequence 15 comprises a number N ofsymbols; the partial sequence 16 a number M of symbols, whereby N and Mcan be the same or different.

In a preferred embodiment the first partial sequence 15 contains pilotsignals, with which the first synchronizing stage of the receiver 6 canbe carried out, in such a way that synchronization firstly takes placewith the symbol clock of the burst 11 and the phase. position of thecarrier frequency of the burst. However synchronization can also firstlybe on the symbol clock of the message and the carrier frequency of theburst. Because this first synchronizing stage is carried out on thereceiver 6 of the base station 2, the second partial sequence 16 can beevaluated more rapidly and reliably for the second synchronizing stage.,since through pre-synchronization synchronization or the firstsynchronizing stage a more rapid and reliable decoding of theinformation contained in the second partial sequence is possible. Asmentioned previously, pilot signals are preferably transmitted in thefirst partial sequence. Symbol sequences are preferably contained in thesecond partial sequence 16 which enable alignment of the time slot ofthe burst and calculation of the channel impulse response. For thispurpose preferably symbol sequences are transmitted, which include atleast two sequences of a pseudo noise. During the second synchronizingstage apart from the channel impulse response (CIR), automatic gaincontrol (AGC) at the receiver 6 can also be calculated. In addition thereceiver 6 can be synchronized with the phase and/or frequency of thecarrier sequence during the first and/or second synchronizing stage orafter the second synchronizing stage.

The terminal stations 3 and the base station 2 contain a protocol unitfor controlling transmission and reception, which protocol unitfunctions according to the ATM/MAC protocol. In addition the terminalstations 3 and the base station 2 include a modem, with which the TDMAbursts 11 can be received and transmitted. In addition intermediatefrequency and high frequency parts known in the art are contained in thetransmitters or receivers of the base station 2 and the terminalstations 3. The channel access control unit (MAC) controls access ofindividual transmitters or receivers to the divided TDMA channel. Inaddition this MAC controller manages the access requests to thetransmission channel as well as the available channel reserves.

For the transmission direction from the terminal stations 3 to the basestation 2 (uplink) the MAC control unit requires transmission reservesfor signalling supervision (signalling overhead). For data communicationeach terminal station 3 and the base station 2 is connected to an ATMinterface. The aforementioned modem by which ATM cells and signallingdata can be received and sent follows this ATM interface. For thispurpose dynamic slot allocation (DSA) is provided for the MAC protocol.

In order to coordinate channel access for the uplink, the time slotsprovided for this are divided into signalling periods. DSA MAC protocolis used for this grouping or division. A signalling period is introducedby a period control PDU in the transmission direction from the basestation 2 to the terminal stations 3. This transmission direction isalso known as downlink. The period control PDU signals the number oftime slots in the next signalling period and the MAC connection whichhas been allocated. A signalling period can contain a variable number ofvery short slots, in particular for the request channel (RQCH).Basically the data packet structures shown in FIGS. 2 b and 2 c shouldbe considered for the MAC control unit. The signal packet structures forthe downlink are shown in FIG. 2 b. FIG. 2 c shows the packet structuresfor the uplink. It is evident that for the downlink only one signalpacket length, for example 53 bytes, is provided. For the uplink howeverat least two packet types are provided. The first, shown in FIG. 2 c atthe top, can comprise a variable length and contain one MACidentification byte (M-Id) and four request channel (RQCH) bytes. Inaddition a number n×53 of ATM cell bytes is provided. Altogether n×is53+5 bytes are therefore transmitted.

The second type of packet structure is shown in FIG. 2 c at the bottom.This data packet only comprises five bytes, whereby one byte for MACidentification (MAC-Id) and four bytes for the request channel areprovided. This type of packet is normally used for random access orpolling mode.

It is therefore clear that the packet structures shown in FIG. 2 c canbe transmitted within the useful data packet 12 in FIG. 2 a. That is tosay the packets shown in FIG. 2 c can be transmitted within the burst11. Of course it would also be conceivable if the packet structuresshown in FIG. 2 b were likewise transmitted in the burst 11 shown inFIG. 2 a. That is to say stepwiselsynchronization of the receiver can beprovided both for the uplink as well as for the downlink. For furtherobservation however only stepwise synchronization of the receiver isdescribed purely as an example on the basis of FIGS. 3 to 11 in the caseof the uplink, FIG. 11 showing a summary overview of the componentsillustrated separately in FIGS. 3 to 10.

According to FIG. 3 the transmitter 8 of a terminal station 3 comprisesseveral sub-assemblies, which are shown as a block diagram. Digitalinformation D is fed to an encoder 17, which is then encoded in theencoder 17. For this purpose the encoder can include a block encoder 18,in which the digital information D is encoded with an (external) blockcode, which serves to correct/recognize errors. In addition the encoder17 can also comprise a convolutional encoder 19, in which the digitalinformation D can be provided with an (internal) code to correct errors.Connected after the encoder 17 is a modulation unit 20, which is alsoknown as a symbol mapper, in which the coded digital information ismodulated according to modulation defaults. Here various modulationmethods are possible: for example QPSK modulation can be used; obviouslyother types of modulation are also conceivable up to 16 APSK modulation.A device 21 is connected after the modulation unit 20, which defines orproduces the frame structure of a burst 11. That is to say the trainingsequence 13 as well as the start and end bits are added to the modulateddigital information, i.e. the useful data packet 12. In addition amultiplexer is provided in the device 21, which allocates the ready togo bursts 11 to the correct time slot. One or more training sequences 13are stored in a storage device 22 for adding the training sequence to auseful data packet 12. Preferably such training sequences are used, thesymbol sequence of which cannot be formed by a useful data packet, sothat useful data packet and training sequence cannot be confused. Atransmission filter 23 is connected after the device 21, which isfollowed by a digital mixer 24, in which a burst 11 is tuned to adigital intermediate frequency ZF. The digital mixer 24 is followed by adigital-analogue converter, from which the now analogue-type signals aretransmitted from the base station 2 via the air interface 4 to thereceiver 6.

The receiver 6 on the input side comprises an analogue-digitalconverter, which is followed by a digital mixer 25, which mixes the nowdigital-type signals with a digital intermediate frequency ZF. In orderto be able to evaluate the message arriving at the receiver 6,appropriate demodulation and decoding devices are provided on thereceiver side corresponding to the encoding and modulation provided onthe transmitter side. In detail these are an input filter 26, which isalso known as a matched or signal-adapted filter, which can beconfigured as a square-root Nyquist filter. Also a demultiplexer 27 isprovided on the receiver side. To recover the original information arecovery device 28 is provided, which includes a Viterbi decoder 29 anda block code decoder 30.

After the analogue-digital converter the digital message is down-mixedto the digital intermediate frequency ZF by means of the diaital mixer25. In order now to be able to carry out demodulation or decoding, asynchronization device 31 is provided which synchronizes the entirereceiver 6 with the incoming message. For this purpose after the inputfilter 26 the training sequence is received by the synchronizationdevice 31, in order to determine at least the clock frequency, thecarrier frequency and/or the carrier phase, so that the components onthe receiver side can be synchronized via synchronizing control lines32. A frequency phase correction device 33 is provided between the inputfilter 26 and the demultiplexer 27 for frequency and/or phasecorrection. By means of the Viterbi decoder 29 tracking can still becarried out after synchronization, in the case of which phase and/orfrequency synchronization can be achieved. The parameters for trackingdefined by the Viterbi decoder 29 can be transmitted to thesynchronizing unit 31 which therefore can be used for postsynchronization of the modules on the receiver side.

The transmitter 8 is partly shown in FIG. 4. Similar or equivalent partsas in the preceding figures are provided with identical referencesymbols. The transmitter-side device with the multiplexer 21 hereincludes an interpolation filter 34, which is followed by a Nyquistfilter with a roll off for example of r=0.3. The Nyquist filter isprovided in FIG. 4 with the reference symbol 35. This filter 35 servesto shape the pulse. The interpolation filter 34 has an interpolationfactor I.

The receiver 6 shown in detail in FIG. 4 comprises the analogue/digitalconverter and the digital mixer 25, to which the digital intermediatefrequency ZF is fed. The input filter 26, which can include a Nyquistfilter 36 and a decimation filter 37, that has a decimation factor d, isalso shown. The synchronization device 31 here comprises in detail apower or energy detector 38, which detects an incoming burst at thereceiver 6. Information from the MAC protocol is fed to this detector38. A sensor device for the carrier frequency, clock frequency or phaseposition of the carrier frequency is also arranged in thesynchronization device 31, which is provided in FIG. 4 with thereference symbol 39. The sensor device 39 determines the appropriatesynchronization parameters from the signal sampled after the filter 36and sends control pulses to the digital mixer 25 and a device 40 forsynchronizing phase or frequency. In addition the signal present afterthe decimation filter 37 is fed to the sensor device 39 and thesynchronization device 40. To produce synchronization the device 40receives information from the Viterbi decoder 29. A phase/frequencycorrection unit or complex multiplier 25′ is arranged between thedecimation filter 37 and the demultiplexer 27, in order to produce phaseor frequency synchronization. A pick-up for a device 41 is providedbetween the decimation filter 37 and the demultiplexer 27, which carriesout channel impulse response and time slot synchronization and sendscorresponding information to the demultiplexer 27. In additioninformation is made available for time slot adjustment and gain control(AGC) from the device 41.

FIG. 5 shows a block diagram for step wise synchronization of thereceiver 6. The received signal D is detected by the power detector 38.Clock and frequency or phase position of the carrier frequency areroughly synchronized during the first synchronizing stage 42. For thispurpose the first partial sequence 15 is evaluated from the receivedsignal D, which is shown by an arrow 43. The demultiplexer 27 issynchronized for phase or frequency and clock by means of the correctingdevice 33.

The second partial sequence 16 is evaluated during the secondsynchronizing stage 45. Transmission of the second training sequence ismarked by the diagram arrow 44. Time slot adjustment and channel impulseresponse (CIR) are evaluated during the second synchronizing stage 45.The receiver is then synchronized for slot adjustment and channelimpulse response by means of a second correcting device 33′.

Tracking T can follow the second synchronizing stage, in which thereceiver is synchronized for frequency and/or phase. The synchronizedreceived signal is then transmitted to the Viterbi decoder 29.

Therefore the parameters necessary for synchronizing the receiver aredetermined in the final stages 42 and 45. Either the clock or frequencyor the clock and the,phase of the received burst are determined duringstage 42, whereby the first training sequence is used for this purpose.The parameters necessary for time slot adjustment and channel impulseresponse are determined in stage 45. In addition the phase can again bedetermined during the second synchronizing stage 45 in particular if theclock and frequency have been determined during the first synchronizingstage. In addition the previously mentioned AGC can also be determined.

According to FIG. 6 the first partial sequence 15, which is shown herepurely as an example by pilot signals (++−−++−− . . . ) is evaluated insuch a way that the first partial sequence 15 received is fed to twolow-pass filters 46 a and 46 b, whereby each filter 46 a and 46 b isfollowed by a linear regeneration unit 47. Clock and frequency offsetare then determined in a calculation unit 48, so that the parametersobtained can be transmitted to the components following the calculationunit 48. The first partial sequence 15 in this case is formed by pilotsignals. The previously mentioned parameters are determined from thesepilot signals. This method is very similar to the known pilot signaltechnology, in which all functional sequences are carried out in thetime domain. Since the first partial sequence 15 includes these pilotsignals + and −, the incoming signal corresponds to a signal withessentially cosine form, so that the Fourier transform of this signalresults in two pilot signals, which contain frequency offset and clockinformation. After the signal has been filtered by the two differentlyconstructed filter elements 46 a and 46 b and as a result of theexecution of linear regeneration in the units 47 clock and frequencyoffset can be deduced. In this embodiment of the first synchronizingstage 1S according to FIG. 6 phase offset is preferably calculatedduring the second synchronizing stage 2S by means of the second partialsequence 16.

In the block diagram according to FIG. 7 firstly the phase offset andclock are determined during the first synchronizing stage (alternativeto the embodiment shown in FIG. 6). For this purpose the first partialsequence 15 is again fed to two filters 46 a and 46 b, whereby thefilter 46 a is followed by a phase detector 49 and the filter 46 b isfollowed by a time detector 50. Phase offset and clock are thendetermined by a mean value calculator 51 and passed onto the devicesdownstream. If preference is given to this variant, frequency offset ispreferably calculated during the second synchronizing stage and thereceiver synchronized with it.

The already pre-synchronized signal is further processed—in accordancewith FIG. 8—for execution of the second synchronizing stage 2S. Thesecond partial sequence 16 is used for time slot synchronization (slotalignment), based on a correlation process. For a fundamental search ormonitoring function, in order to be able to detect all the symbols inthe time slot, the time slot must be evaluated symbol by symbol. Reducedsymbol intervals in this case are considered by the fact that it isknown through the MAC protocol whether a time slot with useful data isconcerned or only a burst with channel request information, whereby theMAC-Id (FIG. 2 c) can be evaluated for this purpose. Within acorrelation range 52 the useful data received is correlated with thesecond partial sequence in a device 53. A parallel or subsequent searchfor maximum excursion (peak) is carried out in a device 54, in order tobe able to produce time slot synchronization in a process step 55. Nextthe time slot received and the second partial sequence 16 are againcorrelated with one another, whereby cross correlation is preferablyused here, as a result of which this takes place through symbols of thefirst training sequence being shifted by a number P in a process step56. As a result the channel impulse response or gain control factor(AGC) can be determined in process steps 57 and 58.

Frequency/phase synchronization is described below on the basis of FIG.9. The incoming signal is delayed in process step 59 by a number ofsamples. Parallel with this non-linear transformation of the incomingsignal takes place in a process step 60. The mean value is determinedfor the entire amount of the samples in a process step 61. Thecalculated phase parameters and the static phase are combined in acombination step 62, whereby the static phase has been determined bycorrelation from the training sequence in a process step 63. Thecombined values are then used for the synchronization position via thecomplex multiplier 25′. In FIG. 9 similar or equivalent parts areprovided as in the remaining figures with identical reference symbols.

Frequency and/or phase synchronization is therefore necessary in manycases since there could be influences from the known jitter and Dopplereffects or similar, as a result of which the phase position can changeover the length of a time slot, in particular in the case of a usefuldata time slot, so that only one-off correction to the appropriate phaseposition with a one-off calculated value is not sufficient in all cases.The bit error rate can therefore be reduced during phase and/orfrequency synchronization. In addition an improvement can be achieved,if a combination of the first and second partial sequences is consideredin such a way that one of the partial sequences includes the pilotsignals, and symbols which provide good correlation results arecontained in the second partial sequence. A Viterbi & Viterbi algorithmwhich is processed in a V & V device (FIG. 11) can be used for phaseand/or frequency synchronization. The parameters provided by this V & Vdevice are then consulted for tracking.

An alternative to the method described in FIG. 9 is clear from the factthat an equivalent input signal D′ (FIG. 10) is reconstructed by meansof the Viterbi decoder 29, whereby a phase comparison between thereceived signal and the reconstructed (rebuilt) signal is subsequentlycarried out. For this purpose an initial value is preferably used, thatis to say one which has been calculated during the first synchronizingstage 1S as phase offset. If the phase offset has been determined duringthe second synchronizing stage, this value can obviously also be used asinitial value. This method is shown as a flow chart in FIG. 10. Similaror equivalent parts as in the remaining figures are provided with thesame reference symbols.

A signal reconstructing device 64 and phase comparator 65 are providedin order to compare the signal acquired after the demultiplexer 27 withthe signal reconstructed from the device 64, in order to calculate thevarious phase position offsets during tracking. The receiver can then besynchronized accordingly with the combination device 62.

1. A method of operating a message receiver for a message that is present as a burst allocated to a time slot which includes at least a training sequence and useful data, said training sequence including at least a first partial sequence and a second partial sequence, and said first partial sequence comprising periodic data, said method comprising stepwise synchronizing said receiver to said burst as said burst is received by performing the steps of: in a first synchronizing stage, using said first partial sequence to determine a clock frequency of said message and at least one of a phase position of a carrier frequency of said burst and the carrier frequency of said burst, and in a second synchronizing stage, using said second partial sequence to synchronize said receiver to said time slot of said burst, said second synchronizing stage occurring after said first synchronizing stage.
 2. The method according to claim 1, and calculating a channel impulse response and a gain control factor (AGC) during the second synchronizing stage.
 3. The method according to claim 1, wherein the receiver is synchronized during the second synchronizing stage with the carrier frequency of the burst.
 4. The method according to claim 1, wherein the receiver is synchronized during the second synchronizing stage with a phase position of the carrier frequency.
 5. The method according to claim 1, wherein the receiver is synchronized with a phase and a frequency of a carrier frequency tracking after the first synchronizing stage or during or after the second synchronizing stage.
 6. The method according to claim 5, wherein the tracking takes place when the message is decoded.
 7. The method according to claim 5, wherein the tracking and decoding take place by means of a Viterbi decoder and according to Viterbi & Viterbi algorithms.
 8. The method according to claim 1, wherein at least one of the two partial sequences includes a symbol sequence, which enables alignment of the time slot of the burst and calculation of channel impulse response, the phase position, gain control factor (AGC) as well as transmission power.
 9. The method according to claim 1, wherein one of the two partial sequences includes pilot signals.
 10. The method according to claim 1, wherein at least one of the two partial sequences includes at least pseudo noise sequences.
 11. The method according to claim 1, wherein the useful data of the message is statistically evaluated for the second synchronizing stage.
 12. The method according to claim 1, wherein the training sequence comprises a constant duration or number of symbols.
 13. The method according to claim 1, and detecting an incoming burst in the receiver by means of power detection.
 14. The method according to claim 1, wherein the time slot has a structure which is determined when the burst arrives.
 15. The method according to claim 1, and calculating a channel impulse response by cross correlation of the burst and the second partial sequence.
 16. The method according to claim 15, wherein, during the cross correlation, the training sequence is shifted by a pre-definable number of symbols, in order to determine a corresponding number of multipath propagation echoes.
 17. The method according to claim 16, wherein the phase position and amplitude of each multipath propagation is determined from the echoes.
 18. The method according to claim 17, and determining the gain control factor from the phase position and the amplitude.
 19. The method according to claim 17, and determining a constant phase position for each of the multipath propagation.
 20. The method according to claim 1, and evaluating a periodic partial sequence in time domain.
 21. The method according to claim 20, and evaluating the second partial sequence within a complex number range.
 22. The method according to claim 1, and adding the training sequence as a preamble to the useful data. 